1. Field of the Invention
The present invention relates to an all-digital frequency synthesizer. More particularly, the present invention relates to an digital frequency synthesizer that consists only simple logics and a linear feedback shift register, which provides lower complexity and superior resolution of frequency.
2. Description of Related Art
Traditionally, either a direct frequency synthesizer, a phase-locked frequency synthesizer, or a digital synthesizer is used to synthesize frequencies. An example of a conventional direct frequency synthesizer is shown in the block diagram in FIG. 2. Providing a frequency 26.7457 kHz is to be synthesized herein, a direct frequency synthesizer utilizes successive approximation to get finer frequency, and thus desired frequency is approached. However, the hardware of this method is obviously complex.
For another example of prior frequency synthesizer, phase-locked technique is applied therein. Referring to FIG. 3, a bock diagram of a phase-locked frequency synthesizer is illustrated. This synthesizer incorporates a frequency divider with a Phase-Locked-Loop (PLL) circuit. The reference frequency, the synthesized frequency, and the minimal resolution in this case are f1, f2, and Δf respectively. This architecture is usually implemented with analog circuitry and is also relatively disorderly as opposed to what is proposed in this present invention.
A digital synthesizer further realizes frequency synthesis function besides direct frequency synthesizer and phase-locked frequency synthesizer. The block diagram of a generic digital synthesizer is shown in FIG. 4. Notice that this architecture is similar to a Numerically Controlled Oscillator (NCO). The reference frequency f1 and the incremental frequency Δθ are predetermined in this system. The synthesized frequency f2 is thus generated via continuously looking up in a cosine-table. However, when a finer resolution is to be synthesized, say 201.3457 kHz, the cosine-table becomes large and this makes the hardware infeasible.
Concluding the three prior frequency synthesizers, complex scheme, analog circuitry, and low capability of target frequency are observed, which are undesirable unfortunately. Thus an all-digital frequency synthesizer complying feasible digital circuitry and finer resolution is proposed in this present invention.